Field of the Invention
The present invention relates to a heat treatment apparatus for heating a sheet precision electronic substrate (hereinafter, simply referred to as a “substrate”), such as a disk-shaped semiconductor wafer, by irradiating the substrate with light.
Description of the Background Art
In the process of manufacturing a semiconductor device, the introduction of impurities is an essential step for forming pn junctions in a semiconductor wafer. It is common at present to introduce impurities by ion implantation and subsequent annealing. Ion implantation is a technique for physically doping a semiconductor wafer with impurities by ionizing impurity elements, such as boron (B), arsenic (As), or phosphorus (P), and causing the impurity elements to collide with the semiconductor wafer with a high acceleration voltage. The doped impurities are activated by annealing. At this time, if the annealing time is about several seconds or longer, the doped impurities may be diffused deeply by heat, and as a result, the junction depth may become too deeper than required and compromise the formation of a good device.
In view of this, flash-lamp annealing (FLA) is recently receiving attention as an annealing technique for heating semiconductor wafers in an extremely short time. Flash-lamp annealing is a heat treatment technique using xenon flash lamps (hereinafter, sometimes simply referred to as “flash lamps”) to irradiate a surface of an impurity-doped semiconductor wafer with flash light and raise the temperature of only the surface of the semiconductor wafer in an extremely short time (e.g., several milliseconds or less).
The radiation spectral distribution of xenon flash lamps ranges from ultraviolet to near-infrared regions, with the xenon flash lamps having shorter wavelengths than conventional halogen lamps, and approximately coincides with the fundamental absorption band of silicon semiconductor wafers. Thus, when xenon flash lamps emit flash light to a semiconductor wafer, less light will pass through the semiconductor wafer and accordingly the temperature of the semiconductor wafer will rise quickly. It is also known that extremely short-time application of flash light, such as several milliseconds or less, will only selectively increase the temperature in the vicinity of the surface of the semiconductor wafer. Such an extremely short-time temperature rise caused by the xenon flash lamps will only activate impurities without deeply diffusing the impurities.
Examples of the heat treatment apparatus using xenon flash lamps include those disclosed in U.S. Pat. No. 4,698,486 and US 2003/0183612, in which desired heat treatment is achieved by a combination of pulsed light-emitting lamps, such as flash lamps, that are arranged on the front side of a semiconductor wafer and continuous lighting lamps, such as halogen lamps, that are arranged on the rear side of the semiconductor wafer. In the heat treatment apparatuses disclosed in U.S. Pat. No. 4,698,486 and US 2003/0183612, a semiconductor wafer is preheated to a certain degree of temperature by, for example, halogen lamps and then heated to a desired processing temperature by pulse heating using flash lamps.
Preheating using the halogen lamps, as disclosed in U.S. Pat. No. 4,698,486 and US 2003/0183612, has a processing advantage that the semiconductor wafer will be preheated to a relatively high preheating temperature in a short time, but at the same time, it may more likely cause a problem that the peripheral portion of the semiconductor wafer will have a lower temperature than the central portion. Conceivable causes of this uneven temperature distribution include heat radiation from the peripheral portion of the semiconductor wafer, and heat conduction into a relatively low-temperature quartz susceptor from the peripheral portion of the semiconductor wafer. In order to address this problem, Japanese Patent Application Laid-Open No 2012-174879 proposes to install a cylindrical louver made of a semitransparent material between halogen lamps and a semiconductor wafer in order to make uniform the in-plane temperature distribution of the semiconductor wafer during preheating.
The presence of the louver, as proposed in Japanese Patent Application Laid-Open No. 2012-174879, ameliorates the problem of a temperature drop in the peripheral portion of the semiconductor wafer, but a new problem arises in that the presence of the louver conversely increases the temperature in a region of a semiconductor wafer that is slightly inward of the peripheral portion of the semiconductor wafer.